In general, wound dressing materials used for healing skin defect regions are required to have the following properties and functions:
1) improved skin compatibility without irritating the skin, PA1 2) flexibility sufficient to expand as the skin moves, and PA1 3) infection controllability.
In order to provide these properties and functions, there have been developed wound dressing materials comprising bio-originated substances such as a pig skin, chitin and collagen or other porous substances.
When a skin defect is caused by a wound, burn, etc., the defect regions are treated by being covered with a conventional wound dressing material as a first aid and then grafting skin from buttocks or other parts to the defect region after the condition thereof is recovered to a level to be further treated.
When a conventional wound dressing material is used, however, the dressing material should be stripped off wherever the defect region is treated.
On all such occasions, new skin, which is regenerating under the dressing material is unfavorably destroyed.
Further, the flexibility of conventional dressing materials comes into question because these materials do not flexibly deform enough following the movement of the skin, which sometimes causes pain.
In the case of porous dressing materials, the infection controllability thereof is also called in question in addition to the flexibility.
As a medical material originating from a protein fiber, silk has been used for a long time as an excellent biocompatible suture.
A permeable membrane which comprises a crystalline substance containing- and water insoluble-fibroin is known as a membrane comprising a protein fiber-originated component (see, Japanese Patent Laid-Open Publication Nos. 63-246,169 and 1-118,545).
A water insoluble fibroin membrane described in Japanese Patent Laid-Open Publication No. 1-118,545 has various uses such as artificial skins, wigs, sweat clothes and the like because of excellent vapor permeability, improved transparency and mechanical strength, and desirable affinity to the human body.
On the other hand, Japanese Patent Laid-Open Publication No. 2-233,128 describes that a crystalline substance containing- and water insoluble-membrane prepared from fibroin and sericin exhibits excellent oxygen permeability, improved transparency and mechanical strength, desirable biocompatiblity and high stability to the human body, and is accordingly useful as contact lenses, artificial skins, blood bags and the like.
Furthermore, Japanese Patent Laid-Open Publication No. 56-40,156 describes that a porous membrane is prepared by blending a water soluble compound such as ethylene glycol, polyalkylene glycol and glycerine to sericin, which exhibits appropriate water absorption properties and vapor permeability required for a skin protecting material, and improved flexibility.
In these crystalline substance containing membranes which comprise water insoluble fibroin, the crystallinity thereof is necessarily at least more than 10%, and preferably more than 15% so as to keep the mechanical strength, while the water content is about 10 to 60% by weight. The crystallinity should be more than 20%, particularly in the case of the above mentioned porous membrane in which the above-mentioned water soluble compound is blended to fibroin.
Fibroin containing permeable membranes of the conventional type, in which the crystallinity is more than 10% to adequately keep the mechanical strength, may be used as artificial skins.
However, these conventional membranes exhibit insufficient flexibility and water absorption properties to use as a wound dressing material for healing a skin defect region, while in the case of porous membranes, the infection controllability is also in question, and accordingly, it is not intended in essence to use such permeable membranes themselves as a wound dressing material.